This invention relates to tunnel junctions.
In recent years, much work has been done on using electron tunneling for cooling applications. The first theoretical investigation of cooling by means of electron tunneling was done to deal with overheating in single electron transistors [A. N. Korotkov, M. R. Samuelsen, S. A. Vasenko, “Effects of overheating in a single-electron transistor,” J. Appl. Phys., 76 (6), p. 3623-3631, (1994)]. Inside the metal/insulator/metal (MIM) tunnel junction, electron tunneling takes place through an insulator layer, but because of the high thermal conductivity of ultra-thin insulator layer, the MIM junctions exhibited large heat backflow, which reduces cooling efficiency. Huffmann (U.S. Pat. No. 3,169,200) attempted a solution to the heat backflow problem, using multiple MIM tunnel junctions connected in series. However, a device of this type could not be fabricated because of its technological complexity. The present inventors have designed tunnel junctions of metal/vacuum/metal (MVM) type, having very low heat backflow [A. Tavkhelidze, G. Skhiladze, A. Bibilashvili, L. Tsakadze, L. Jangidze, Z. Taliashvili, I. Cox, and Z. Berishvili, Proc. XXI International Conf. on Thermoelectric, August 26-29 IEEE, New York, pp. 435-438 (2002); L. B. Jangidze, A. N. Tavkhelidze, M. O. Tetradze and T. N. Devidze, “Methods for improving surface flatness in thick Cu film electrodeposition”, Russian Microelectronics, Vol. 36 (2), p. 116, (2007)]. Such tunnel junctions may be used for efficient cooling. A theoretical investigation of MVM tunnel junctions has shown that the cooling coefficient could be as high as 20-30% [Y. Hishinuma, T. H. Geballe, B. Y. Moyzhes, and T. W. Kenny, Appl. Phys. Lett. 78, 2572 (2001); T. Zeng, Appl. Phys. Lett. 88, 153104 (2006)]. An approach using emission from semiconductor resonant states across a vacuum gap has also been proposed [A. N. Korotkov and K. K. Likharev, Appl. Phys. Lett. 75, P. 2491-2493 (1999)].
Most cooling applications require tunnel junctions with a large area—of the order of square centimeter and more. The electrodes for tunnel junctions should be flat within few Angstroms to allow fabrication of uniform vacuum nano gap. Available polishing methods allow fabrication of surfaces with local roughness of 0.2 nm. However gradual deviation in the surface relief over large distances is as high as 500 nm per centimeter. The local roughness (0.2 nm) is low enough to obtain local vacuum tunneling, but because of a gradual deviation in the surface relief, it becomes impossible to bring large areas of two electrodes (polished independently) close enough to each other.